Unidirectional condenser microphones include a front acoustic terminal for capturing sound waves from a sound source on the front side of a diaphragm and a rear acoustic terminal for capturing the sound waves on the back side of the diaphragm, and take advantage of acoustic resistance to adjust bi-directional components captured from the rear acoustic terminal.
The value of acoustic resistance on the rear acoustic terminal side is designed based not only on a required directivity (for example, cardioid, hypercardioid, supercardioid and the like) and stiffness in an air chamber present on the rear of the fixed pole, but also on the distance between the front and rear acoustic terminals (see “Analysis of the Miniaturization of Directional Condenser Microphone” by Mizoguchi Akio, Journal of The Acoustical Society of Japan, Vol. 31, No. 5, 1975, FIG. 1, in particular).
In normal unidirectional condenser microphones, since the distance between acoustic terminals is at most 5 cm or less, a relatively low acoustic resistance value is used.
In contrast, narrow directional microphones with an acoustic tube attached to a microphone unit thereof may have the distance between acoustic terminals that may reach as long as 50 cm at a low frequency.
Consequently, the front side of the diaphragm of the condenser element is subject to an acoustic mass in the acoustic tube, and in order to achieve unidirectivity, such as hypercardioid, a considerably high acoustic resistance value is needed on the rear acoustic terminal side. In addition, for precise directivity, fine tunability is requisite while such a high acoustic resistance value is maintained.
In a condenser microphone, the fixed pole is supported on an insulator. In a microphone of unidirectional type, a perforated electrode plate is used for the fixed pole and sound holes are drilled in the insulator as sound wave passages such that sound waves from the rear acoustic terminal act on the back side of the diaphragm. Moreover, an air chamber is present between the insulator and the fixed pole. The insulator is therefore an important element for designing an acoustic resistance value.
Meanwhile, since the fixed pole is connected to a gate of a field-effect transistor (FET) that serves as an impedance converter through an electrode feedthrough rod inserted through the insulator, the insulator is composed of a material that has high volume and surface resistivities, for example, polycarbonate.
Even such a highly resistive material may undergo surface resistivity degradation because the material is more or less prone to brush marks and small cracks when it is cut to produce an insulator.
Injection molding is not likely to impose such a problem as with cutting. A normal unidirectional condenser microphone with a short distance between acoustic terminals and a narrow directional microphone with an acoustic tube, however, require a different number of sound holes with a different diameter drilled in the insulator in their connection with required acoustic resistance values.
Furthermore, even microphones of the same narrow directional type have a different number of sound holes with a different diameter drilled in the insulator in their connection with the acoustic resistance values depending on the length of the acoustic tube used. This entails a significant cost burden in order to produce a molding die for each model of microphone.
Accordingly, an object of the invention is to provide an insulator, which supports a fixed pole in a unidirectional condenser microphone and is preferably injection molded, in a shareable manner among microphones that are different in the distance between acoustic terminals from one another.